The field addressed by the invention is characterized by intensive ongoing maintenance activity due to frequent changes in setups and severe wear and tear on the wiring, cables, power outlets, interconnection receptacles, lamps and related equipment. Certain electrical testing requirements which arise frequently and recurrently fall into two groups:
(1) off-line testing where there is no powerline voltage present: (a) verifying the correct hookup, isolation and general integrity of a typical three-wire extension cable by making continuity tests between all three contact members of both end connectors and (b) when only the plug end is available for testing while the socket end is connected to a load, such as a lamp, verifying the integrity of the load and any intervening cable(s) by a continuity test between the hot and neutral contact members at the plug end; and PA1 (2) on-line testing: testing outlet sockets for the presence of a.c. powerline voltage between the correct socket contact members in either (a) a fixed outlet socket or (b) an outlet socket at the end of a cable.
It is always possible to perform such testing with general-purpose electrical measuring equipment such as voltmeters and ohmmeters on an impromptu basis using such expedients as clip leads; however, such testing tends to be slow, laborious, and erroneous, and in on-line testing, lack of adequate insulating safeguards, e.g. the use of so-called "suicide cords", may cause a power outage if a short circuit trips a breaker, and the operator may be exposed to the risk of lethal or dangerous shock.
It has been known to organize such testing by providing customized test apparatus with a suitable mating socket and/or plug, including means for attaching suitable measuring instruments, for example as taught by Arlow in U.S. Pat. No. 3,718,859.
As a step further, it has been known to provide apparatus including some form of switching or selection means lo to facilitate several different continuity tests. Such apparatus as is presently known and available, including ohmmeters, voltmeters, and continuity testers, may be adaptable to address one or other aspect of various testing requirements separately, but not in the combination sought by the present invention.
Heretofore, the combination of off-line and on-line testing addressed by the present invention has typically required the use of at least two separate pieces of test apparatus and setups. Even if the various pieces of apparatus were to be combined in a single enclosure, there is a serious problem of isolating the continuity testing circuitry from the risk of damage from unintented presence of powerline voltage. One approach would be the use of two separate test connector plugs, one for the off-line continuity tests and the other for on-line voltage tests; however, since these plugs would have identical pinout configuration there would be a risk of unintentionally connecting the wrong plug to a powered outlet thus applying the powerline voltage directly to the continuity tester, heavily overloading the powerline as well as destroying the continuity tester. Other approaches such as switching schemes which rely on the user's vigilance, warning signs, etc., do not offer a "fail-safe" solution.
Thus there remains an unfulfilled need for a specialized single convenient combination continuity and line voltage tester, fitted with a single test plug and a test socket, to provide convenient, rapid, safe and unambiguous testing, both on-line and off-line, of electrical equipment such as a.c. powerline sources, cables, lamps and other loads as found in the practice of studio and stage lighting and the like.